When water is hypoxic, the low amount of oxygen can severely damage, and even kill, hundreds to thousands of fish at once.

Fish die-offs that result from hypoxic conditions are well-documented in warm, coastal waters such as the Gulf of Mexico. But similar events can occur globally in both marine and freshwater.

For decades, scientists in southeast Alaska have observed significant die-offs of salmon in rivers before the fish were able to spawn. Some scientists suspected that hypoxia was the culprit behind these events, but it wasn’t obvious what would cause hypoxic conditions in Alaskan streams in the first place.

That’s because hypoxia is most frequently observed in warm water with lots of algae and rapidly decomposing organic matter-very different from the conditions in cold, clear, Alaskan streams where salmon spawn.

Ecologist Christopher Sergeant of the National Park Service set out to investigate if hypoxic events were to blame for salmon die-offs in Alaskan streams and, if so, what caused the hypoxia.

He teamed up with research fish biologist Ryan Bellmore of the USDA Forest Service Pacific Northwest (PNW) Research Station, graduate student Casey McConnell of the University of Alaska Fairbanks, and associate professor of aquatic ecology and conservation Jonathan Mooreof Simon Fraser University to investigate these questions.

A central focus of their study was to determine if the dense populations of salmon themselves significantly contributed to the formation of hypoxic conditions.

The researchers used dissolved oxygen probes to measure and record data in the Indian Riverand Sawmill Creek of southeastern Alaska. This resulted in a valuable, high-frequency, long-term data series on seasonal hypoxic conditions. They also measured water temperature, river flow, and did periodic visual counts of salmon.

The scientists defined the hypoxic threshold in this study as 7 mg oxygen per liter, Alaska’s minimum level of dissolved oxygen in streams with salmon. Previous studies found that salmon demonstrated poor swimming performance and delayed migration below this level.

By measuring dissolved oxygen, the scientists recorded four hypoxic events between 2010 and 2015 that lasted for very different lengths of time (1.5 hours to 37 days). Two of these events resulted in documented salmon die-offs.

River flow levels and salmon densities in southeast Alaska are both influenced by changes in climate and direct human intervention. River flows are expected to decrease in the future as climate patterns shift the balance of precipitation and snow melt. In addition, humans directly increase pressure on river ecosystems by diverting water for a variety of purposes and artificially increasing salmon spawning densities through hatchery strays.

Together, these factors point to the likelihood of more hypoxic events in the future. More hypoxic events could increase die-offs of spawning salmon and other fish in these streams, including cutthroat trout and Dolly Varden.

“Hypoxia is another mechanism by which a changing climate may influence salmon,” says PNW Research Station’s Ryan Bellmore who coauthored the study. “Additional research is needed to understand which populations may be most at risk.”

Going forward, hypoxia poses an invisible, but very real, threat to spawning salmon populations. However, the observations and results of this research provide the kind of information that fisheries managers need to properly manage salmon populations in the future.